1. Field of the Invention
The present invention relates to piezoelectric ceramic compositions and piezoelectric elements. More specifically, the present invention relates to a piezoelectric ceramic composition suitable for uses requiring a high piezoelectric strain constant d31, and to a piezoelectric element, particularly a monolithic piezoelectric element, using the piezoelectric ceramic composition and which is used for piezoelectric devices, such as piezoelectric actuators, buzzers, and sensors.
2. Description of the Related Art
Monolithic piezoelectric elements using a piezoelectric ceramic composition have been widely used for piezoelectric actuators, piezoelectric buzzers, piezoelectric sensors, and the like.
The piezoelectric ceramic composition needs to have a high piezoelectric strain constant d31, and Pb{(Ni, Nb), Ti, Zr}O3 materials (hereinafter referred to as PNN-PZT) are known as piezoelectric ceramic compositions having a high piezoelectric strain constant d31 and which comprise lead zirconate titanate (hereinafter referred to as PZT) containing a third constituent Pb(Ni, Nb)O3.
Also, this type of piezoelectric ceramic composition needs to have a high mechanical strength to prevent fracture and cracking during manufacture. Accordingly, a PNN-PZT piezoelectric ceramic composition having a high piezoelectric strain constant d31 and a high mechanical strength has been proposed in, for example, Japanese Patent No. 3119101.
In the known art, the Ni/Nb content ratio is reduced to be lower than the stoichiometric ratio, that is, 1/2, to reduce the size of the particles constituting the ceramic in order to enhance the mechanical strength,.
Piezoelectric elements using the piezoelectric ceramic composition, which are used for various piezoelectric devices, such as piezoelectric actuators, buzzers, and sensors, require a high piezoelectric strain constant d31. However, other quality requirements, such as thermal resistance, depend on the intended use of the piezoelectric devices. For example, some piezoelectric elements may give the thermal resistance a higher priority than the piezoelectric strain constant d31, and others may give the piezoelectric strain constant d31 a higher priority than the thermal resistance.
In any case, the need for further enhanced piezoelectric characteristics, such as a high piezoelectric strain constant d31, arises in order to achieve a piezoelectric device having much higher quality.
In the manufacture of a monolithic piezoelectric element, ceramic sheets having an internal electrode pattern formed of a Agxe2x80x94Pd conductive paste are layered on one another and contact-bonded to form a laminate, and the laminate is sintered to obtain a piezoelectric ceramic base element. Since the ceramic sheets and Ag are, therefore, sintered together, the Ag is undesirably diffused into the ceramic base element to negatively affect piezoelectric characteristics in the use of the known piezoelectric ceramic composition.
Accordingly, an object of the present invention is to provide a piezoelectric ceramic composition capable of achieving desired piezoelectric characteristics according to use, and to a piezoelectric element using the piezoelectric ceramic composition.
In general, it is desirable to implant ions into a PNN-PZT piezoelectric ceramic composition so that the content of donor ions Nb5+ is higher than that of the acceptor ions Ni2+, in order to stably obtain a high piezoelectric strain constant d31. However, an implantation of too much excess Nb is liable to lead to the formation of a heterogeneous phase during calcination of a ceramic powder mixture. Accordingly, the amount of implanted Nb needs to be limited to some extent from the viewpoint of the stability of characteristics of the ceramic powder material.
The inventors of the present invention have found, as a result of intensive research, that the piezoelectric strain constant d31 of a PNN-PZT piezoelectric ceramic composition can be further increased by implanting Nb, acting as the donor, excessively to some extent and by reducing the Pb molar content by a predetermined amount from the stoichiometric ratio. Also, by setting the component ratios of the piezoelectric ceramic composition in a predetermined range leading to relatively good piezoelectric characteristics, and by reducing the Pb molar content from the stoichiometric ratio, the piezoelectric characteristics can be enhanced more effectively.
To this end, according to one aspect of the present invention, a piezoelectric ceramic composition expressed by the formula Pbxcex1[{Niw/3Nb1xe2x88x92(w/3)}xTiyZrz]O3 is provided, wherein x, y, and z lie on the lines connecting points A(0.10, 0.42, 0.48), B(0.10, 0.48, 0.42), C(0.40, 0.39, 0.21), and D(0.40, 0.33, 0.27) in a ternary diagram or within the region surrounded by the lines, w satisfies the relationship 0.85xe2x89xa6w less than 1.00, and xcex1 is lower than the stoichiometric ratio.
By reducing the Pb molar content xcex1 by a predetermined amount from the stoichiometric ratio, Ni in the composition is precipitated in grain boundaries during firing. Consequently, the Ni/Nb ratio in the composition decreases to increase the Nb content to excess, and, hence, the donor content is made still higher. As a result, the piezoelectric strain constant d31 can be increased without the formation of a heterogeneous phase during calcination.
However, an excessively reduced Pb molar content xcex1 negatively affects sintering characteristics due to a large difference from the stoichiometric ratio. Accordingly, the Pb molar content xcex1, preferably, satisfies the relationship 0.950xe2x89xa6xcex1xe2x89xa60.995. Alternatively, part of the Pb may be replaced with at least one element selected from the group consisting of Sr, Ca and Ba. Preferably, the amount of the Pb replaced with those elements is about 10 mole percent or less, but not 0 mole percent.
By replacing about 10 mole percent or less of Pb with Sr, Ca or Ba, the donor content is increased to excess, and, consequently, the piezoelectric strain constant d31 can be increased, as with the foregoing composition.
According to another aspect of the present invention, a piezoelectric element is provided which comprises a ceramic base element comprising the piezoelectric ceramic composition and an internal electrode disposed in the ceramic base element.
By reducing the Pb molar content xcex1 of the piezoelectric ceramic composition to be lower than the stoichiometric ratio, as described above, Ni is further precipitated in grain boundaries to compensate for the diffusion of Ag or other internal electrode constituents into the ceramic base element. As a result, the donor content of the piezoelectric ceramic composition is further increased, so that the deterioration of piezoelectric characteristics due to the internal electrode constituents can be canceled out. Thus, the resulting monolithic piezoelectric element can exhibit excellent piezoelectric characteristics.
The piezoelectric characteristics can be enhanced according to the present invention, by setting the B site variables so as to be suitable for use, and/or by reducing the Pb molar content from the stoichiometric ratio. Thus, the resulting piezoelectric element can exhibit high performance and is suitably used for various types of piezoelectric devices, such as piezoelectric actuators, piezoelectric buzzers, and piezoelectric sensors.